Productivity or injectivity of a well relates to the wellbore radius. The larger the wellbore radius, the better the productivity or infectivity. However, drilling a larger borehole could be prohibitive because of substantial increase of drilling and completion cost for a larger borehole. For a weak or unconsolidated formation, it would be beneficial to enlarge the wellbore by producing sand to some extent before fracture packing and other gravel packing operations. Perforating in such weak or unconsolidated sand formations often induces collapse of the perforation tunnels and even the near wellbore formation. Hence, the perforation naturally allows sand production from the formation for enhancement of the productivity or injectivity. However, conventional perforation in weak or unconsolidated sand also results in sand accumulation in the wellbore. The produced sand in the wellbore can clog the gun and complicate the completion operations. For example, sand control and other completion devices may not be able to be positioned at the right place before the sand in the wellbore is completely cleaned out. Therefore, although producing some sand from formation through perforations may increase the well productivity and infectivity, it is beneficial not to produce any sand into the wellbore after perforation.
Except for sand production from the perforation in weak or unconsolidated formation, debris in the perforation tunnels for consolidated formation is also detrimental for well productivity and injectivity. Dynamic underbalanced perforating techniques, disclosed in U.S. Pat. No. 6,554,081, U.S. Pat. No. 6,598,682, U.S. Pat. No. 7,121,340 and U.S. Pat. No. 7,182,138, can be very efficient to remove the crushed zone near the wall of the perforation tunnels and clean the debris in the perforation tunnels out of formation. However, for weak or unconsolidated sand formation, dynamic underbalance perforating can actually sometimes make the sanding worse. Without proper control, the produced sand could lead to the failure of the completion operations.
Hence, it is desirable to have a better perforating technique in weak or unconsolidated formation.